An optical fiber and a bundle of optical fibers are variously modified in accordance with the conditions under which they are used. However, a tension member must be used with an optical fiber to assure a high strength in some cases. When water permeates an optical fiber cable, its strength may be degraded. When an optical fiber cable is to be installed in the bottom of a sea or the bottom of the water, in order to assure a sufficient installation tension and a high water resistance, an optical fiber cable must be used in a jacket structure in which an optical fiber cable is covered with a thin metal tube.
In this manner, there is provided an apparatus and method of continuously manufacturing a metal tube covered optical fiber cable, as disclosed in Published Unexamined Japanese Patent Application No. 64-35514.
This apparatus for manufacturing the metal tube armored optical fiber cable forms a continuously fed flat metal strip into a metal tube having a longitudinal gap at a top portion. A guide tube is inserted into the metal tube through this gap of the metal tube, and an optical fiber is guided into the metal tube through the guide tube. After the gap of the metal tube having received this optical fiber is closed, the metal tube is supplied to a laser welding unit.
The laser welding unit causes a guide roller to align the abutment edge portions of the top portion of the metal tube to each other. A laser beam having a focal point at a position outside the range of the abutment portions is radiated to weld the abutment portions. Since the laser beam is focused outside the range of the abutment portions, the abutment portions can be welded without protecting the optical fiber with a heat-shielding member.
This metal tube containing the optical fiber cable is drawn to have a predetermined outer diameter, and the drawn tube is continuously wound around a capstan.
During drawing of this metal tube, an inert gas is supplied to the guide tube to carry the optical fiber cable by the viscosity resistance of the gas. While the metal tube is kept engaged with the capstan, the optical fiber cable is blown outward against the inner surface of the metal tube, so that the length of the optical fiber cable is set larger than that of the metal tube. The optical fiber cable is not kept taut to prevent the optical fiber cable from stress caused by an installation tension or the like.
In order to protect the optical fiber cable from water entering from a hole formed in a damaged metal tube, a gel is injected inside the metal tube. More specifically, after the optical fiber cable is blown outward against the inner surface of the metal tube by the inert gas at the capstan, the gel is injected from a gel guide tube different from the guide tube for guiding the optical fiber cable.
Optical fiber cables are used in a variety of application conditions and at various temperatures. The thermal expansion coefficient of the metal tube is much larger than that of the optical fiber cable. For this reason, when optical fiber cables are used at high temperatures, a tension acts on the optical fiber cable due to a difference in elongations of the metal tube and the optical fiber cable to damage the optical fiber cable. This also occurs when a cable is installed at a high tension, e.g., in installment at the bottom of a sea.
To the contrary, when optical fiber cables are used at low temperatures, the optical fiber cable is brought into contact with the inner wall surface of the metal tube having a large shrinkage amount due to a large difference between the degrees of shrinkage of the metal tube and the optical fiber cable. The optical fiber cable directly receives a side pressure from the inner wall of the metal tube. Irregular bending forces having short periods act on the optical fiber cable to cause a so-called microbend loss, thereby attenuating a signal transmitted through the optical fiber cable.
In order to prevent damage and the like, the optical fiber cable is blown outward against the inner wall surface of the metal tube while the metal tube is kept engaged with the capstan, so that the length of the optical fiber cable is set larger than that of the metal tube after the cable is straightened for use.
In this case, however, a difference between the length of the optical fiber cable and the length of the metal tube (to be referred to as an extra length hereinafter) is determined by the outer diameter of the capstan and a difference between the inner diameter of the metal tube and the outer diameter of the optical fiber cable. The extra length cannot be arbitrarily controlled, and the optical fiber cable may be damaged depending on application conditions.
As described above, while the metal tube is kept engaged with the capstan, the optical fiber cable is blown outward against the inner surface of the metal tube by an inert gas to provide an extra length to the optical fiber cable. For this reason, when a gel is to be injected into the metal pipe, it must be injected while the optical fiber cable is kept blown outward against the inner wall of the metal tube due to the following reason. That is, even if the gel is injected and then the inert gas is supplied, the gel causes resistance to fail to give the extra length to the optical fiber cable. When a gel is to be injected, a gel guide tube is required in addition to the optical fiber cable and the inert gas guide tube. Since these two guide tubes must be simultaneously inserted into the metal tube, the inner diameter of the metal tube is increased. In order to obtain a thin tube from this tube, the drawing amount is increased. Metal tubes may not be occasionally thinned in accordance with diameters of optical fiber cables, resulting in inconvenience.